In JP-A-7-268638, there is proposed a plating process which comprises carrying out a plating while filtering a plating solution with the aim of preventing a body to be plated from a deposition of sodium oxalate precipitate formed in an electroless copper plating solution.
In JP-A-61-183474, there is mentioned a technique of using glyoxylic acid as a reducing agent in electroless copper plating solution. In this document, it is mentioned that NaOH or KOH is used for alkalifying the pH of electroless copper plating solution, and especially that KOH is more desirable than NaOH because potassium oxalate is higher than sodium oxalate in solubility of an oxalate salt which is an oxidized product of glyoxylic acid.